Research
Radiographic Evidence of Non­occupational Asbestos Exposure
from Processing Libby Vermiculite in Minneapolis, Minnesota
Bruce H. Alexander,1 Katherine K. Raleigh,1 Jean Johnson,2 Jeffrey H. Mandel,1 John L. Adgate,1,3
Gurumurthy Ramachandran,1 Rita B. Messing,2 Tannie Eshenaur,2 and Allan Williams 2
1Division
of Environmental Health Sciences, School of Public Health, University of Minnesota, Minneapolis, Minnesota, USA; 2Minnesota
Department of Health, St. Paul, Minnesota, USA; 3Department of Environmental and Occupational Health, Colorado School of Public
Health, Aurora, Colorado, USA
Background: Community exposure to asbestos from contaminated vermiculite ore from Libby,
Montana, occurred in many processing sites in the United States, including a densely populated
urban residential neighborhood of Minneapolis, Minnesota.
Objective: We examined exposed community residents who never worked at the plant or never lived
with a plant worker for radiographic evidence of lung changes consistent with asbestos exposure.
Methods: We obtained posteroanterior chest radiographs to identify the prevalence of pleural
abnormalities consistent with pneumoconiosis, as determined by consensus of two National
Institute for Occupational Safety and Health–certified B-reader radiologists. We estimated cumulative asbestos exposure (fibers per cubic centimeters × months) with air dispersion model data and
activity-based modeled exposure estimates for vermiculite processing waste contact. We modeled
associations between pleural abnormalities and asbestos exposure using multiple logistic regression
to adjust for year of birth, sex, and potential occupational asbestos exposure.
Results: Radiographs were obtained for 461 participants. The prevalence of pleural abnormalities
by B-reader consensus was 10.8%. A history of direct contact with the waste and ever playing in the
waste piles was associated with pleural abnormalities {odds ratio [OR] 2.78 [95% confidence interval (CI): 1.26, 6.10] and 2.17 (95% CI: 0.99, 4.78), respectively, when adjusted for background
exposure}. The regression coefficients for log-transformed measures (fibers per cubic centimeters ×
months) of background exposure and activity-based exposure were 0.322 (95% CI: 0.078, 0.567)
and 0.063 (95% CI: –0.013, 0.139), respectively, when adjusted for each other, and 0.283 (95%
CI: 0.104, 0.463) for cumulative exposure from all sources.
Conclusion: These results support the hypothesis that community exposure to asbestos-contaminated vermiculite originating from Libby, Montana, is associated with measurable effects based on
radiographic evidence.
Key words: amphibole, asbestos, community exposure, Libby vermiculite, lung diseases, ­pleural
abnormalities. Environ Health Perspect 120:44–49 (2012). http://dx.doi.org/10.1289/ehp.1103529
[Online 12 October 2011]
Vermiculite ore from a mine near Libby,
Montana, is known to be contaminated with
amphibole asbestos (primarily winchite, richterite, and tremolite), which is liberated in
the mining and processing of the ore [Agency
for Toxic Substances and Disease Registry
(ATSDR) 2008; Meeker et al. 2003]. The
mining and processing of vermiculite in Libby
have been associated with elevated mortality from lung cancer, non­malignant respiratory disease, and mesothelioma (Amandus
and Wheeler 1987; McDonald et al. 1986a;
Sullivan 2007) in exposed workers, members
of the community, and workers employed at
distant vermiculite processing sites (Amandus
et al. 1987; McDonald et al. 1986b; Peipins
et al. 2003; Rohs et al. 2008). Environmental
contamination from this source is not limited
to Libby, as hundreds of thousands of tons of
the ore were shipped throughout the country
for processing. One of the destinations for
the Libby vermiculite ore was the Western
Minerals/W.R. Grace (WM/WRG) facility in
Minneapolis, Minnesota.
The WM/WRG facility processed vermiculite ore mined in Libby, Montana,
44
from 1938 to 1989. This facility was one of
approximately 250 such processing facilities
in the United States that received Libby ore.
At the WM/WRG facility, the vermiculite
ore was heated in two furnaces and expanded
in a process known as exfoliation to make
Zonolite® insulation, Monokote® fireproofing, and other building materials. In addition
to the vermiculite product, the exfoliation
produced a waste material that contained
up to 10% amphibole asbestos [Minnesota
Department of Health (MDH) 2005]. The
waste rock was piled on the WM/WRG
property and offered freely to the community. A unique aspect of the WM/WRG exfoliation site in Minneapolis is its location in
a residential neighborhood: a mix of singlefamily homes, multifamily homes, schools,
and churches. Neighborhood residents hauled
the rock from the piles and used it for gardening and as fill material for driveways and
yards. Neighborhood children also played on
the piles of vermiculite processing waste, as
access to the site was not restricted. The proximity of the neighborhood to the plant and
the opportunities for contact with the waste
volume
rock created a scenario for substantial non­
occupational exposure to asbestos.
Responding to residential concern in 2000
(Gordon 2000), the MDH and ATSDR initiated the Northeast Minneapolis Community
Vermiculite Investigation (NMCVI). The
NMCVI staff surveyed > 1,600 properties
and worked with the U.S. Environmental
Protection Agency (EPA) to document
contamination on > 260 properties, which
subsequently were cleaned by the U.S. EPA
in a time-critical removal under the federal
Superfund program. The MDH/ATSDR
investigation assessed past and present Libby
asbestos exposure pathways for workers and
community members, including relative fiber
levels and exposure duration (MDH 2005)
in addition to estimating ambient air exposures during the years the plant was operating
(Kelly et al. 2006; MDH 2003). Interviews
were conducted with > 6,400 community
members to establish baseline information. As
a follow-up to the exposure characterization
in this community, we evaluated the impact
of non­occupational exposure to asbestos from
contaminated vermiculite on lung health as
evidenced by radiographic changes in the
lungs of community members.
Materials and Methods
Approval for the study protocol was obtained
from the human subjects committees of the
University of Minnesota and the MDH. All
participants were guided through an informed
consent process, and written consent was
obtained before their participation.
Study population. The focus of this study
was non­occupational exposure to asbestos
from contaminated vermiculite in the population surrounding the WM/WRG site in
Address correspondence to B.H. Alexander, Division
of Environmental Health Sciences, University of
Minnesota, School of Public Health, Room 1260,
Mayo Building, 420 Delaware St. SE, Minneapolis,
MN 55455 USA. Telephone: (612) 625-7924. Fax:
(612) 625-4936. E-mail: [email protected]
We thank J. Korinek, N. Pengra, D. Kampa, and
R. Hoffbeck for assistance with this research.
This work was supported by Agency for Toxic
Substances and Disease Registry grant 1 R01
TS000014.
The authors declare they have no actual or potential
competing financial interests.
Received 6 February 2011; accepted 12 October
2011.
120 | number 1 | January 2012 • Environmental Health Perspectives
Community asbestos exposure from Libby vermiculite
Minneapolis. We identified 2,222 members of
the original NMCVI cohort who were never
employed at the WM/WRG plant, were not
a household contact of WM/WRG employees, and were first at risk of exposure to the
contaminated vermiculite before 1980. The
latter inclusion criterion ensured an adequate
latency period for effects of the asbestos exposure to become apparent.
To obtain a sample representative of the
range of community exposures, the population was stratified into groups to represent
three exposure scenarios: a) intense intermittent exposure, b) long-term high ambient
background exposure, and c) low ambient
background exposure. We classified people
with a childhood history of playing in the
piles of waste rock outside the plant as the
group with intense intermittent exposures
to potentially high concentrations of asbestos fibers. The long-term high and low background exposure groups were selected based
on residential history and frequency matched
to the age distribution of the intense intermittent exposure category. All members of the
cohort were classified by cumulative exposure experienced before 1 January 1980. The
exposure estimates were based on the periodspecific air dispersion model developed by
the Minnesota Pollution Control Agency and
the MDH and on residential history. The
initial model estimated exposure based on the
amount of time the individual lived in the
affected area multiplied by the yearly average
plant emissions rates (Kelly et al. 2006; MDH
2003). Excluding persons identified as having
intense intermittent exposure and those who
did not have appropriate residence data, a
total of 1,839 individuals were classifiable by
background exposure. The long-term high
background exposure group was initially identified as the upper quartile of this exposure
distribution. The low-exposure category was
selected from the lowest quartile of this distribution. When the available samples from the
upper and lower quartile were exhausted, the
second and third quartiles were entered into
the selection protocol. This selection protocol
ensured a distribution of estimated exposure.
The institutional review board protocol
required MDH, as the institution of record
for the NMCVI, to initiate contact. MDH
forwarded the names and contact information of individuals interested in learning
more about the study to the University of
Minnesota. The potential partici­pants were
sent a letter and study brochure, along with
consent and HIPAA (Health Insurance
Portability and Accountability Act) forms.
This was followed with a call from a trained
telephone interviewer to review the study purpose, procedures, requirements, consent, and
HIPAA forms and to answer any questions.
Those who chose to participate signed and
returned the consent and HIPAA forms to
the study office.
Upon receiving the signed consent and
HIPAA forms, a second packet containing a
cover letter, clinic information, and questionnaire was mailed to each participant. An interviewer followed-up by telephone to ensure the
instructions and procedures were understood
and to encourage the participant to promptly
attend a participating clinic and return the
completed questionnaire.
Radiographic measurements. Participants
attended a local contract medical clinic that
specializes in occupational health screening and could perform plain-film X-rays for
B-reading. The clinics offered evening and
weekend appointments to accommodate various schedules. The prevalence of pleural abnormalities, parenchymal opacities, and other
evidence of asbestosis was determined with a
single posteroanterior (PA) radiograph following the recommendations of the International
Labour Organization (ILO 2002).
The radiographs were reviewed and scored
by National Institute for Occupational Safety
and Health–certified B-readers using standard
methodology (ILO 2002). The B-readers were
board-certified, practicing radiologists from a
practice in the Minneapolis–St. Paul metropolitan area. The presence of pleural abnormalities consistent with pneumoconiosis,
both pleural thickening and pleural plaques,
was the primary indicator of asbestos-induced
changes in the lungs. The final classification of
radiographic changes consistent with pneumoconiosis required consensus by two B-readers.
If there were discrepancies between two initial readers regarding pleural abnormalities
or the presence of parenchymal abnormalities
with an ILO classification of 1/0 or above, the
radiographs were reviewed by a third certified
B-reader. The clinical interpretations were
summarized and sent to the participants and
their health care provider if requested.
Exposure assessment. We assessed exposure to asbestos fibers through ambient exposure and activities that brought participants in
contact with the waste rock. The details of the
exposure assessment are summarized here and
reported in full elsewhere (Adgate et al. 2011).
Briefly, background exposure was determined
by length of residence in the affected community and estimates of airborne fiber concentration. The MDH study ascertained residential
histories within the affected community for
all study participants (Kelly et al. 2006). Air
dispersion modeling and deposition were used
to characterize cumulative potential asbestos background exposure for this population
(Kelly et al. 2006; MDH 2005). The models
evaluating facility emissions and resulting air
concentrations and deposition accounted for
prevailing weather patterns, emission controls installed in the plant, and post­-shutdown
Environmental Health Perspectives • volume 120 | number 1 | January 2012
emissions due to fugitive dust and construction activities. Ambient fiber concentrations during peak production years before
installation of air pollution control equipment (1938–1972) varied by several orders
of magnitude, with concentrations ranging
from 0.026 fibers/cm3 (f/cc) within one to
two blocks of the plant to 0.0001 f/cc at the
margins of the neighborhood. The exposures
for this study were estimated for the period
of plant operation (1938–1989). Cumulative
background exposure (CBG; fibers per cubic
centimeter × months) was estimated by summing duration and period-specific airborne
fiber concentration for each subject:
CBGi = Σ BGijTj ,
[1]
where BGij is monthly average airborne asbestos fiber concentration (fibers per cubic centi­
meter) for subject i from the air dispersion
model during the jth time period, and Tj is
the number of months subject i lived in the
impact zone of WM/WRG plant emissions.
Potential activity-based exposure pathways were ascertained by questionnaire for
the NMCVI study. The interviews solicited
self-reports of direct contact from moving waste
rock from WM/WRG plant, using waste rock at
home (e.g., on their lawns or gardens), installing
or removing vermiculite insulation, and playing
in or around waste piles at the plant. Exposures
were estimated based on probable ranges of the
activity durations, the frequency of the activities,
and ranges of potential airborne fiber concentrations. The potential airborne fiber concentrations were derived from experimental data
that reconstructed these activities in the U.S.
EPA Libby investigation (Weis 2001a, 2001b).
Uniform distributions were assumed based on
the following ranges for each activity: moving
waste rock from plant, 0.07–0.14 f/cc; using
waste rock at home, 0.02–0.227 f/cc; installing or removing vermiculite insulation,
0.142–0.568 f/cc; and playing in or around
waste piles at the plant, 0.14–1.72 f/cc. Monte
Carlo simulation software (Crystal Ball 2000,
version 2.2; Decisioneering, Boulder, CO) was
used to estimate the probability distribution of
asbestos exposures (fibers per cubic centimeter
× months) for each activity (k) based on the
assumption that during these activities each
individual was exposed to a range of asbestos
concentrations with varying exposure durations
that ranged from 0.25 to 8 hr. The cumulative
activity concentration (CAC; fibers per cubic
centimeter × months) for each activity (k) was
estimated as follows:
CACik = (AC × T × F × 12)
÷ (24 × 365),
[2]
where for each individual i, AC is activityspecific asbestos concentration in air (fibers
45
Alexander et al.
per cubic centimeter ), T is exposure duration
(hours/day), and F is frequency the activity
occurred (total days). The expression ACk ×
Tk × Fk has units of fibers per cubic centimeter × hours. The remainder of the expression [12 months/year ÷ (24 hr/day × 365
days/year)] converts the result to fibers per
cubic centimeter × months.
The total cumulative asbestos exposure
(TCE; fibers per cubic centimeter × months)
for each individual is the sum of the estimated
background levels and the median values of
the simulation output distribution for each
individual:
TCEi = Σk CBGi + CACik .
[3]
To focus on the community exposure, the
population for this study explicitly excluded
employees of WM/WRG and their household
members. However, other occupations also
have the potential for asbestos exposure. To
characterize this potential exposure, we used
responses from the NMCVI questionnaire that
asked about ever employment in 15 occupations that have potential for asbestos exposure.
Statistical analysis. The prevalence odds
ratios (ORs) of pleural changes were modeled
with multivariate logistic regression for the
following exposure metrics: background exposure, total activity-based exposure, exposure
acquired as a child when playing on the waste
piles, and total exposure. We used cumulative exposure metrics, expressed as fibers per
cubic centimeter × months. The exposure data
were log-normally distributed, and model fit,
as determined by likelihood ratio statistics,
was improved by using the log-transformed
variable. We categorized the exposure data
as < 50th, 50th–75th, and > 75th percentiles
for background, total exposure, and activity
exposure for those with any activity, respectively. The models were adjusted for sex, year
of birth, and ever holding a job with potential
asbestos exposure as reported on the NMCVI
questionnaire. Missing exposure data were
imputed with a multiple imputation algorithm (SAS MI procedure, version 9.2; SAS
Institute Inc., Cary, NC) based on the exposure data for all eligible persons (n = 2,222),
creating 18 replicate imputed data sets. We
imputed missing data for activity-based exposures for only those individuals reporting the
activity. For descriptive purposes, we summarized the imputed data with the median
value for each parameter. Logistic regression
models using the 18 imputed data sets were
summarized using the MIANALYZE procedure in SAS (version 9.2). Prevalence ORs
are reported to describe the associations, and
95% Wald’s confidence intervals (CIs) are
presented to characterize the precision of
the estimates.
Yes
461
No
1,761
made for 10.6% of the participants. Seven
participants had X-rays of insufficient quality
for B-reads; thus, the final prevalence of consensus abnormalities was 10.8%. Consensus
readings of diffuse pleural thickening and
pleural plaques were reported in 5 and 45 participants, respectively. Parenchymal abnormalities equal to or exceeding ILO category 1/0
were recorded by at least one B-reader in 18
participants, and 9 of these had the consensus
of two readers. One of these had a film coded
as unreadable by one B-reader, so 8 were clearly
ILO ≥ 1/0. There were too few parenchymal
abnormalities to statistically evaluate, but of
the 8 with a consensus of ≥ 1/0, all had some
evidence of exposure through an activity, 6 of
which reported playing in the waste piles. The
8 individuals were distributed across the background and total exposure estimates, with 3, 2,
and 3 in each of the upper three quartiles. Half
of the parenchymal abnormalities also reported
having a job with potential asbestos exposure.
Participants diagnosed with pleural thickening or pleural plaques were older and more
likely to be male, to have held an asbestosexposed job, and to report activities associated with contaminated vermiculite contact
(Table 3). The exposure distribution for background exposure and activity-based exposure
followed a pattern of higher exposure in those
with pleural abnormalities.
In models adjusting for age, sex, and
history of any asbestos-exposed job, the
log-transformed measures of background
exposure, exposure from pile playing, total
activity exposure, and total exposure were
positively associated with pleural abnormalities (Table 4). When both background and
activity-based metrics were in the model, the
background exposure maintained the strongest association with pleural abnormalities.
The OR for a difference equivalent to the
interquartile range (IQR) of total exposure
(IQR = 0.014–0.245 = 0.231 f/cc-months)
was 2.22 (95% CI: 1.34, 3.68).
When evaluated categorically, the association between pleural abnormalities and background exposure appeared limited to exposure
at or above the 75th percentile (Table 5).
Persons reporting ever having contact with
241 (52.3)
220 (47.7)
860 (48.8)
901 (51.2)
Table 2. Summary of radiographic results for all
participants attending the clinic (n = 461).
89 (19.3)
139 (30.2)
148 (32.1)
85 (18.4)
242 (52.5)
180 (39.0)
51 (11.1)
44 (9.5)
44 (9.5)
223 (48.4)
129 (28.0)
654 (37.1)
374 (21.2)
385 (21.9)
348 (19.8)
1,040 (59.1)
401 (22.8)
126 (7.2)
109 (6.2)
129 (7.3)
569 (32.3)
479 (27.2)
Results
From the original 2,222 persons eligible, 1,765
were randomly selected from the exposure
groups and were eligible for contact by the
MDH, and 1,133 assented to be contacted by
the University of Minnesota. Of the remaining 632, 68 were deceased, 367 declined, and
197 could not be reached. A total of 698 consented to participate, and 461 completed the
clinic visit. Compared with non­participants,
those who attended the clinic were more likely
to be male, born after 1940, and non­smokers
and to have activities that would lead to exposure (Table 1). The percentage of participants
and non­participants who reported having an
asbestos-exposed job was essentially the same.
Playing on the waste piles was reported by 180
(39%) of the 461 participants, and 223 (48%)
reported at least one activity resulting in exposure to the waste material.
Prevalence of pleural abnormalities consistent with pneumoconiosis reported by any
B-reader was 15.4% (Table 2). A consensus
determination of pleural abnormalities was
Table 1. Characteristics of the original eligible study population (2,222 persons) who did and did not
attend the clinic [n (%)].
Attended clinic
Characteristic
Total
Sex
Male
Female
Birth year
≤ 1940
1941–1950
1951–1960
≥ 1960
Ever a regular smoker
Played on waste piles at plant
Moved waste from WM/WRG plant
Used waste at home, lawn, or garden
Installed/removed vermiculite insulation
Any activities leading to exposure
Ever held any asbestos-exposed job
46
volume
Radiograph interpretation
Any pleural abnormalitya
Pleural abnormality,a consensus of
two readers
Diffuse pleural thickening, consensus of
two readers
Any pleural plaques observed
Pleural plaques, consensus of two readers
Any parenchymal abnormalityb
Parenchymal abnormality,b consensus of
two readers
aConsistent
n (%)
71 (15.4)
49 (10.6)
5 (1.1)
70 (15.2)
45 (9.8)
18 (3.9)
9 (2.0)
with pneumoconiosis. bAt ILO ≥ 1/0.
120 | number 1 | January 2012 • Environmental Health Perspectives
Community asbestos exposure from Libby vermiculite
the waste rock or ever playing on waste piles
had ORs of 3.60 (95% CI: 1.71, 7.58) and
3.23 (95% CI: 1.59, 6.52), respectively. When
background exposure was added as a covariate,
ORs decreased to 2.78 (95% CI: 1.26, 6.10)
and 2.17 (95% CI: 0.99, 4.78), respectively.
When examined by exposure percentiles,
the strength of association increased in the
unadjusted models, but a monotonic exposure response beyond the 50th percentile was
not evident in the adjusted models. The ORs
for the third and fourth quartiles, compared
with exposures below the median, were also
attenuated when the background exposure
was accounted for. When all sources of exposure were combined, the prevalence ORs were
3.79 (95% CI: 1.76, 8.16) and 1.05 (95%
CI: 0.44, 2.50), respectively, for the fourth
and third quartiles of exposure compared with
exposures below the median.
Discussion
We observed an association between the prevalence of pleural abnormalities and estimates
of environmental exposure to asbestos fibers
attributed to processing Libby vermiculite.
These results support the hypothesis that non­
occupational community exposure to asbestos
fibers from the processing of Libby vermiculite produced measurable effects. Associations
were most consistent with long-term, lower
exposure, as represented by background exposure, in contrast to more intense but intermittent exposure from specific activities.
These results must be considered in context with the following limitations of the
study. The study is small, which limited estimate precision and our ability to explore more
complex exposure models. The requirement of
attending a clinic for an X-ray, with no explicit
benefit to the participant, likely limited study
participation, and it is possible that participation was influenced by known health status.
However, arguments may be constructed to
favor a positive or negative bias in the results.
Our metrics of activity-based exposure were
derived from self-reported activities as assessed
in 2001 when the cohort was enumerated.
Although this provided excellent baseline
information for the whole cohort, assumptions about the duration of the activity were
needed, which likely introduced some exposure misclassification. An air dispersion model
was used for retrospective estimation of background exposure based on residential history
and proximity to the vermiculite plant. This
model assumes an equivalent exposure opportunity for all people, regardless of where they
spent their day. Assessment of radiographic
pleural abnormalities was accomplished with a
single PA chest radiograph, and although this
is a standard screening practice, pleural abnormalities may be missed without evaluating
multiple views. More sensitive assessments of
pleural abnormalities can be done with computed tomography (CT) scans; however, cost,
potentially unnecessary exposure to ionizing
radiation, and risk of frequent false-positive
findings reduce the utility of CT scans for
screening low-risk individuals. False-positive
misclassification was reduced by accepting
pleural abnormalities only when there was consensus by two B-readers. The initial assessment
of the two primary readers detected 71 pleural
abnormalities consistent with pneumoconi­osis,
whereas the consensus readings only found 49.
In a number of the disputed assessments of
pleural abnormalities ultimately classified as
normal, the clinic notes from the dissenting
B-reader postulated the presence of pleural
shadows from fat. Associations were attenuated
if the 22 participants with disputed readings
were classified as having pleural abnormalities. For example, the regression coefficient
for background exposure was reduced from
0.401 (95% CI: 0.165, 0.637) to 0.146 (95%
CI: –0.091, 0.382), and cumulative exposure
was reduced from 0.283 (CI: 0.104, 0.463) to
0.168 (95% CI: 0.014, 0.323).
An estimated 138,000 tons of Libby
ore were processed at the WM/WRG plant
in Minneapolis. The resulting emissions and
subsequent use of the waste rock contaminated
areas up to 2 miles from the plant (Kelly et al.
2006). Our exposure models combined ambient dispersion and activity-based estimates
that demonstrate substantial non­occupational
asbestos exposures for residents of this neighborhood, albeit with considerable variability
by exposure pathway (Adgate et al. 2011).
Although median exposure estimates for the
cohort were driven by direct plant emissions,
contact with waste rock resulted in substantial additional exposures to some individuals.
Most important, playing on piles of waste rock
was the largest contributor to high-end exposures for the subpopulation of individuals who
engaged in these activities.
Previous research in Libby has shown that
playing on piles of waste rock is associated
with changes in lung function and is a risk
factor for asbestos-related disease (Horton
et al. 2006, 2008; Peipins et al. 2003).
Although these studies of retrospective community exposure to asbestos explored the relationship between Libby asbestos and various
health effects, most of them did not provide
individual estimates of cumulative exposure.
Our estimates of cumulative exposure concentrations (fibers per cubic centi­meter × months)
Table 3. Characteristics of study participants with and without pleural abnormalities.
Pleural abnormalitiesa
Characteristic
Totalb
Sex
Male
Female
Birth year
≤ 1940
1941–1950
1951–1960
≥ 1960
Played on waste piles
Moved waste from WM/WRG plant
Used waste at home, lawn, or garden
Installed/removed vermiculite insulation
Any contact with the waste
Any activities leading to exposure
Any asbestos-exposed job
Total exposure, f/cc × months [median (IQR)]
Background exposure [median (IQR)]
Activity-based exposure [median (IQR)]
Yes
49 (100)
No
405 (100)
39 (79.6)
10 (20.4)
198 (48.9)
207 (51.1)
19 (38.8)
20 (40.8)
8 (16.3)
2 (4.1)
28 (57.1)
13 (26.5)
8 (16.3)
8 (16.3)
35 (71.4)
33 (67.3)
22 (44.9)
0.202 (0.032–0.480)
0.079 (0.023–0.161)
0.013 (0.0–2.72)
67 (16.5)
116 (28.6)
140 (34.6)
82 (20.2)
147 (36.3)
37 (9.1)
34 (8.4)
35 (8.6)
182 (44.9)
167 (41.2)
105 (25.9)
0.049 (0.012–0.202)
0.028 (0.008–0.066)
0.0 (0.0–0.079)
Values are n (%), except as noted.
aConsensus by two B-readers. bExcludes 7 cases with X-rays that could not be classified by B-readers.
Table 4. Regression coefficients from logistic regression models for associations between prevalence of
pleural abnormalities and log concentration of background, activity, and total exposure.
Model
1
2
3
4
5
6
aAdjusted
Parameter
Background exposure
Total activity exposure
Cumulative exposure
Background exposure
Total activity exposure
Pile-playing exposure
Pile-playing exposure
Background exposure
β-Estimatea ± SE (95% CI)
0.401 ± 0.120 (0.165, 0.637)
0.098 ± 0.036 (0.028, 0.168)
0.283 ± 0.091 (0.104, 0.463)
0.322 ± 0.125 (0.078, 0.567)
0.063 ± 0.039 (–0.013, 0.139)
0.080 ± 0.030 (0.021, 0.139)
0.042 ± 0.034 (–0.025, 0.109)
0.331 ± 0.130 (0.076, 0.585)
p-Value
0.001
0.006
0.002
0.009
0.102
0.008
0.221
0.011
for year of birth, history of having an asbestos-exposed job, and sex.
Environmental Health Perspectives • volume 120 | number 1 | January 2012
47
Alexander et al.
do not directly translate to the existing healthbased standards. The current Occupational Safety
and Health Administration (OSHA) permissible
exposure limit, which has been reduced considerably in the last 40 years (ATSDR 2001), is
0.1 f/cc based on an 8-hr time-weighted average
(OSHA 1995). The estimated median asbestos
concentrations in U.S. cities range from 10–8
to 10–4 f/cc (ATSDR 2001). At 10–4 f/cc the
range of cumulative expo­sure from 20 to 40
years would be 0.024–0.048 f/cc-month, which
is in the lower end of the exposure distribution
we describe. The median concentration for the
participants with pleural abnormalities was an
order of magnitude greater. Thus, exposure estimates developed for this study were well above
urban background. When converted to a scale
of fibers per cubic centimeters × year, the upper
end of the exposure distribution for this study
was within the lower quartile exposure ranges
in which pleural abnormalities were observed
in workers in Libby (Horton et al. 2008; Rohs
et al. 2008).
The impact of Libby asbestos exposure
on the health of employees of the Libby vermiculite mines and mills has been well documented. Mortality studies of W.R. Grace
mine workers in Libby have found significantly increased mortality from nonmalignant
respiratory disease and lung cancer (Amandus
and Wheeler 1987). A more recent update
of an occupational cohort of 406 vermiculite
miners employed before 1963 and followed
through 1999 reported an excess of lung cancer deaths [44 deaths; standardized mortality
ratio (SMR) = 2.40], non­malignant respiratory deaths (51 deaths; SMR = 3.09), and
all-cause deaths (285 deaths; SMR = 1.27),
which increased with cumulative exposure
(McDonald et al. 2004). A total of 12 mesothelioma deaths had occurred by the end of
1999, representing 4.2% of 285 deaths in
the cohort since July 1983. These associations
continue to be corroborated through additional follow-up of this working population
(Sullivan 2007). Health effects from occupational exposures to Libby asbestos in operations beyond Libby are emerging. Workers at
a vermiculite expansion plant that produced
material primarily for lawn care products were
found to have increased risk of pleural abnormalities at exposures lower than those seen in
Libby (Rohs et al. 2008).
Beginning in 2000, ATSDR began investigating the effects of occupational and non­
occupational exposure to Libby asbestos in
the Libby community. A study of death certificates from 1979 through 1998 found a
40- to 60-fold elevation in asbestosis deaths
in Libby (Horton et al. 2006). These deaths
occurred primarily among former employees.
A respiratory health screening investigation
Table 5. Number of pleural abnormalities (n = 49) and prevalence OR estimates by categorical exposure
level for background exposure, activity-based exposure, and total exposure [OR (95% CI)].
Exposure
Any contact with waste
No (referent)
Yes
Ever played on piles
No (referent)
Yes
Activity exposure (f/cc × months)c
< 0.082
0.082 to < 0.422
≥ 0.422
Pile-playing exposure (f/cc × months)c
< 0.158
0.158 to < 0.549
≥ 0.549
Background exposure (f/cc × months)d
< 0.034
0.034 to < 0.077
≥ 0.077
Total exposure (f/cc × months)d
< 0.0523
0.0523 to < 0.245
≥ 0.245
Sex
Female
Male
Exposed job
No
Yes
n
Unadjusteda
Adjustedb
14
35
1
3.60 (1.71, 7.58)
1
2.78 (1.26, 6.10)
21
28
1
3.23 (1.59, 6.52)
1
2.17 (0.99, 4.78)
32
9
8
1
2.04 (0.83, 5.00)
2.36 (0.92, 6.05)
1
1.34 (0.52, 3.44)
1.37 (0.49, 3.83)
36
6
7
1
1.45 (0.53, 3.98)
2.49 (1.75, 9.41)
1
0.97 (0.34, 2.77)
1.31 (0.44, 3.85)
16
8
25
1
0.91 (0.36, 2.31)
3.18 (1.53, 6.59)
1
0.85 (0.32, 2.11)
2.52 (1.15, 5.50)
17
11
21
—
—
—
1
1.05 (0.44, 2.50)
3.79 (1.76, 8.16)
10
39
—
—
1
3.8 (1.6, 8.9)
22
27
—
—
1
1.31 (0.63, 2.73)
aUnadjusted
for corresponding exposure covariates. All models are adjusted for year of birth, sex, and history of an
asbestos-exposed job. bAdjusted for corresponding exposure. Activity-based exposure is adjusted for background
exposure, and background exposure is adjusted for total activity exposure. cCut points are based on the 50th and 75th
percentile of those with any activity. All participants without activity are included in the referent category. dCut points
are at the 50th and 75th percentile of the exposure distribution.
48
volume
conducted in Libby, Montana (Peipins et al.
2003), found radiographic pleural and interstitial abnormalities associated with occupational exposure among former W.R. Grace
employees. Among 365 employees screened,
51% had pleural abnormalities and 4%
had interstitial abnormalities as determined
by at least two B-readers. Among community members in Libby, the rates of pleural abnormalities were higher among males
and increased with age, smoking history, and
body mass. Increased prevalence of pleural
abnormalities was also found among female
household contacts of W.R. Grace employees
(OR = 3.62; 95% CI: 2.70, 4.83) compared
with females who were not household contacts. Participants living > 34 years in the
Libby area also were at increased risk (OR =
2.12; 95% CI: 1.66, 2.70) compared with
persons living < 14 years in the area. People
who played on vermiculite waste piles frequently had twice the prevalence of pleural
abnormalities as those who never played in
the piles (OR = 2.02; 95% CI: 1.59, 2.57).
Persons exposed through multiple exposure
pathways were also at increased risk. Further
evidence (Whitehouse et al. 2008) of potential non­occupational health effects in Libby
has recently been documented and includes
31 cases of mesothelioma that had no documented occupational exposure to asbestos
but were exposed to emissions from the plant
in Libby. A report (Whitehouse 2004) of 123
Libby patients with occupational and non­
occupational exposure indicated progressive
loss of pulmonary function associated with
pleural changes in 94 individuals.
Environmental and secondary occupational exposures to asbestos have been associated with malignant mesothelioma and benign
pleural disease in several settings (Ferrante
et al. 2007; Reid et al. 2008, 2009). Although
these results support the notion that non­
occupational asbestos exposure has hazardous consequences, the exposures encountered
were much higher than seen in our study.
Low-level persistent exposure to asbestos in other populations also has been
associated with measurable changes in the
lungs. “Endemic pleural plaques” have been
described in areas of the world where mineral fibers occur naturally in soil or are used
locally for whitewashing (Ates et al. 2010;
Karakoca et al. 1998; Metintas et al. 2005;
Yazicioglu 1976; Yazicioglu et al. 1980).
We report a stronger association between
the prevalence of pleural changes and estimated cumulative exposure acquired over
many years compared with higher intermittent peak exposures. The reason for this is
not clear. It may speak to the scale of exposure estimates from activity-based exposure
or an overestimate of the frequency of exposure through activities, such as pile playing.
120 | number 1 | January 2012 • Environmental Health Perspectives
Community asbestos exposure from Libby vermiculite
Alternatively, the fiber particles from persistent long-term, lower-level exposure from
fugitive plant emissions, although correlated
with the activity-based exposure, may be sufficiently different from the exposures in the
waste rock that a greater proportion of the
dust is deposited in the lower reaches of the
respiratory tract. Our study, however, was
too small to discern these differences.
In summary, we observed a consistent
association between environmental asbestos
exposure from the processing of Libby vermiculite and prevalent pleural abnormalities.
These results suggest the effects of asbestos
exposure may occur at lower levels than
previously believed. Future research should
concentrate on the effects of long-term, lowlevel exposure and adverse outcomes in non­
occupational, asbestos-exposed populations.
Correction
In the “Results” section of the manuscript
originally published online, OR (95%
CI) values were transposed in two places:
a) for persons reporting ever having contact
with the waste rock or ever playing on the
waste piles, and b) for the same categories
when background exposure was added as a
covari­ate. Data were presented correctly in
Table 5. The text has been corrected here.
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